The present invention relates to a reduction method for correcting silver images comprising dots or/and lines, which are obtained by developing silver halide photosensitive materials, particularly a photosensitive material for plate-making, after exposure. More particularly, the present invention relates to a reducer which has superior reducibility and excellent shelf stability and which causes no yellow stain on a photosensitive material after it has been treated therewith.
There are many literatures and patents concerning reduction of silver images, particularly dot or line silver images formed on a photosensitive material for plate-making, and various reducers have heretofore been used. In the old days, the Farmer's reducer, containing potassium ferricyanide as the principal component, was a common reducer. Recently, however, a cerium sulfate reducer and a ferric ethylenediaminetetraacetic acid chelate reducer have been used from the viewpoint of environmental pollution.
As a photosensitive material for plate-making, so-called lithographic film has mainly been employed. However, new high-contrast photosensitive materials, which contain a hydrazine compound or a tetrazolium compound, have recently been employed. Therefore, the required reducer characteristics have also changed.
The Farmer's reducer is neutral and has excellent reduction characteristics, but it involves a pollution problem and is inferior in keeping property. The cerium sulfate reducer is excellent in keeping property and has relatively good reducibility, but it is strongly acidic and hence inferior in handling properties, and it suffers from the disadvantage that a yellow stain is likely to be left in the non-image area. The ferric ethylenediaminetetraacetic acid chelate reducer is neutral and less costly, but it is inferior in reducibility. With a new high-contrast photosensitive material of high silver iodide content, a yellowish brown residue occurs around the silver image, and the effective reduction range decreases. In addition, a yellow stain is left in the image area after the reduction process.
The expression "excellent reducibility" as used herein means that the ratio of the decrease in area of the silver image to the lowering in density of the silver image is high, that is, the reduction range is wide.
Japanese Patent Application Laid-Open (KOKAI) No. 1-282551 (1989) discloses a reducer containing ferric 1,3-diaminopropanetetraacetic acid chelate, as a reducer that solves the above-described problems. However, this reducer has been revealed to be disadvantageous in that with passage of time, a precipitate separates out, and the reducibility lowers.
Further, since the conventional ferric aminopolycarboxylic acid chelate is generally weak in oxidizing power, the rate of reduction (oxidation) thereof is low in comparison to cerium sulfate, potassium ferricyanide, etc. Therefore, it is difficult to form a practical reducer by using the conventional ferric aminopolycarboxylic acid chelate. Even if reduction is carried out for a long time with a compound containing such material, the dot density lowers to a substantial degree, and a yellow stain occurs in the image area after the reduction process, particularly when it is used in combination with the above-described new high-contrast photosensitive material. In addition, it is likely that a yellowish residue will be left around each dot.
Further, Japanese Patent Application Laid-Open (KOKAI) No. 52-68419 (1977) discloses a reducer that uses a heterocyclic mercapto compound, particularly a combination of such a compound and ferric ethylenediaminetetraacetic acid chelate. However, this combination cannot solve the above-described problems, either.
On the other hand, the finished condition of the plate-making material often needs correction at the customer's request. It is a common practice to use a reducer for correction of a local area. The reducing operation is usually conducted on a light table where water is flowing (i.e., a washing light table). Since the reducer is light yellow, even if the reducer flows out to an area other than a predetermined area, the operator is likely to be unaware of it and let a necessary portion to disappear undesirably. Hitherto, such a problem has often occurred.
Further, care must been taken when a plate-making material subjected to a reduction process is to be printed on a press plate because the image density in the reduced area has been lowered due to oxidation of silver. A problem often arises from this, particularly when the reducing process and the press plate making process are carried out in different places.
However, the conventional reducer containing ferric aminopolycarboxylic acid chelate as an oxidizing agent causes a yellowish brown residue to be left around each dot, so that the reduced area cannot be distinguished from the other area by a visual observation, although it can be done by an observation with a microscope.
It is a first object of the present invention to provide an improved reducer, which is free from the above-described problems.
It is a second object of the present invention to provide a reducer and a reduction method, which are particularly suitable for the above-described new high-contrast photosensitive materials.
It is a third object of the present invention to provide a reducer of excellent working properties, which enables perception of the outflow of the reducer and the reduced area in particular.